The present invention relates to an electrochemical cell (battery) with a metal container. More particularly, the present invention relates to seal members for sealing the active materials and electrolyte within the cell.
Many electrochemical cells have metal containers. The container may be in the form of a can, with a closed bottom end and an open top end, or in the form of a tube, open at both the top and the bottom. The open end(s) of the container may be closed with a cover (often called a cup in button cells). The cover may be attached directly to the edge of the container, e.g., by laser welding. However, it is common for a seal member, made of a thermoplastic material, to be disposed between the container side wall and the cover or cup to form a compressive seal.
The seal member in a typical primary cylindrical consumer alkaline battery also forms a seal around a current collector that extends through a central aperture in the seal member to make electrical contact between one of the electrodes inside the cell and an external contact terminal. Such thermoplastic seal members are popular in consumer batteries because they are relatively easy and inexpensive to manufacture, the cells are easy to assemble, and the resultant seal is acceptable for most consumer applications. Primary alkaline battery seal members may also include a pressure relief mechanism, such as a thinned or otherwise weakened section that will rupture or become displaced to release internal pressure from inside the cell and prevent bursting of the cell.
The seal member in a typical consumer button cell and in a typical consumer cylindrical lithium battery is a gasket in the form of an annular ring with an L-shaped or J-shaped cross section. A wall at the periphery of the gasket is located between the inner surface of the cell container (or can) and an outer surface or edge of the cell cover (or anode cup). The edge portion of the container is reduced in diameter and/or crimped inward and downward to compress the gasket wall between the container and the cover to seal the cell.
Battery manufacturers are continually trying to improve the seal members for electrochemical cells by improving on those characteristics that limit the seal members in some way. Electrochemical cells must remain sufficiently sealed over long periods of time and under a broad range of temperature and relative humidity conditions in order to have satisfactory shelf life and perform as expected after shipping and storage. For this reason, the material of the seal member must remain highly stable. Stress relaxation, creep, thermal stability, tensile modulus, flexural modulus and toughness are all characteristics that reflect aspects of the stability of thermoplastic materials, and some of these characteristics can be affected by temperature and/or humidity. Since the seal member is generally exposed directly to the internal environment of the cell, it must also be stable in that environment. This means it must not deteriorate in contact with the electrolyte or electrode materials. The rate of transmission of electrolyte solvents and certain other liquids and gases must be sufficiently slow to prevent excessive loss of electrolyte and wasteful corrosion reactions within the cell.
In order for a seal member to maintain a compressive seal against other cell components over a long period of time, it is desirable to minimize the rate of stress relaxation in the thermoplastic seal material. To reduce the rate of stress relaxation, fillers such as talc, calcium carbonate, carbon black, silica and the like have been added to the seal member material. However, even when this is done, the stress relaxation rate may still be higher than desirable. Mineral fillers also tend to be distributed non-uniformly after molding, which can lead to non-uniform seal member properties, defects and cell sealing deficiencies.
Some seal members perform additional functions. For example, some have rupture vents for releasing internal pressure from the cells. Some material characteristics will also affect how well the seal member performs these additional functions.
A common method of manufacture of thermoplastic seal members is injection molding. Some problems and limitations of such seal members are related to the injection molding process. For example, it is important that the material fill the mold cavities completely, without unintended voids, and uniformly, without creating weak weld lines. Weak weld lines are formed when surfaces of material filling a cavity meet but do not flow and melt together completely. The molded seal member preferably is uniform in density and other material characteristics. Molded parts may fracture at weak weld lines when the parts are put under tension. Molds can be designed and molding parameters can be adjusted to minimize weak weld lines and other problems related to filling of the mold cavities, but the characteristics of the material being molded also play a vital role in the quality and sealing characteristics of the molded parts.
A wide variety of materials are known as suitable for injection molded thermoplastic seal members for electrochemical cells. Polypropylene, polysulfone, and, in particular, nylon are popular for aqueous alkaline cells.
However, nylon has several major disadvantages. First, it absorbs moisture, making it necessary to dry the nylon prior to molding. After molding, the dimensions and properties of the resulting seal member are also affected by the tendency of nylon to absorb moisture.
Nylon is also subject to hydrolytic degradation when it comes in contact with aqueous alkaline electrolyte. Hydrolytic degradation of nylon occurs through chain scission of amide bonds. This chain scission embrittles the material, leading to seal failure and leakage from the cell. To help prevent this, protective coatings are often used on the surfaces of the seal member that can come in contact with the aqueous electrolyte. In particular, nylon seals for aqueous alkaline cells are generally provided with a coating of asphalt. Application of this asphalt coating involves additional process steps and materials, thus adding to the manufacturing cost of the seal member and the cell.
Another problem with nylon seals for alkaline cells is that they have a relatively high ultimate elongation. For safety reasons, seal members for consumer alkaline cells larger than button cells are typically designed to provide controlled release of pressure in the event that the internal cell pressure increases beyond an acceptable limit. This is often achieved by forming the seal member with a relatively thin portion, which is designed to rupture if the internal pressure increases beyond an established limit. Sufficient space must be provided within the cell to allow the thin portion to extend and rupture. Under normal moisture conditions, nylon may extend to over 300% of its initial length. This high level of elongation requires large amounts of internal cell space, limiting the seal and cell design, and, therefore, the internal volume available for active materials.
Another disadvantage of nylon seal members for electrochemical cells is that the physical properties of the seal, e.g., its strength, are dependent upon the moisture content of the nylon. The moisture content is dependent upon the relative humidity of the environment in which the cell is stored. Accordingly, the vent pressure (i.e., the pressure at which the thin portion of the seal member ruptures) is undesirably dependent on relative humidity.
Seal members made of polypropylene can soften extensively at the high end of the temperature range at which consumer cells may be used (75-85° C.). This results in lower deflection temperatures under load and excessive stress relaxation in the compressive sealing zones of the seal, leading to electrolyte leakage and unreliable cell performance.
The use of polysulfone as a material for electrochemical seal members has been relatively limited due to its relatively high cost. Polysulfone also has a tendency to absorb moisture, and it must be dried to less than about 0.02% moisture before it can be injection molded into a seal member.
Other materials, alone and in various combinations are also known for making seal members for electrochemical cells. Many of these were developed to overcome problems or deficiencies with previously known materials. For example, U.S. Patent Application Publication No. 2001/0014419 A1 discloses a seal for a galvanic cell formed of a styrenic polymer blend including a styrenic polymer and an impact modifying agent which increases the toughness of the styrenic polymer. The disclosed seal has many advantages over conventional galvanic cell seals, including improved chemical resistance to alkaline medium, low water absorption, low coefficient of linear thermal expansion, good heat resistance at high pressures, quick cooling after molding, low melt viscosity, relatively low tensile strength, high impact toughness, relatively high heat deflection temperature, relatively low elongation to break, and high hydrogen permeability. However, even these seal members have shortcomings. While the melt viscosity of the disclosed impact modified styrenic polymer is relatively low, it is still higher than desirable. This can result in improper flow of material into the mold during injection molding, and, in the manufacture of seal members for conventional alkaline cells such as shown in FIG. 1, producing weak weld lines. While mold designs can be changed and molding parameters can be adjusted to minimize the formation of weld lines, there are practical limits in doing so, and such changes can create other problems and/or add to the cost of manufacturing the seal members. It may also be possible to modify the seal member material, e.g., by blending ingredients, but the number of possibilities for doing so are enormous.
Accordingly, an object of the present invention is a thermoplastic injection molded seal member for an electrochemical cell, the seal member being made from a material that has improved molding characteristics, resulting in improved quality, reduced dimensional variability, and lower manufacturing scrap.
A further object of the invention is a seal member with improved sealing characteristics when used in an electrochemical cell.
Another object of the invention is an electrochemical cell with improved shelf life, performance and reliability during manufacturing, shipment, storage and use of the cell.